Category: Fish

In Southwestern France, a group of fish have learned how to kill birds. As the River Tarn winds through the city of Albi, it contains a small gravel island where pigeons gather to clean and bathe. And patrolling the island are European catfish—1 to 1.5 metres long, and the largest freshwater fish on the continent. These particular catfish have taken to lunging out of the water, grabbing a pigeon, and then wriggling back into the water to swallow their prey. In the process, they temporarily strand themselves on land for a few seconds.

For corals, gardening’s a matter of life and death. Corals compete with algal seaweeds for space, and many types of seaweed release chemicals that are toxic to corals, act as carriers for coral diseases and boost the growth of dangerous microbes. These dangers require close contact—the seaweed poisons won’t diffuse through the water, so they need to be applied to the corals directly. And that gives the corals an opportunity to save themselves. When they sense encroaching seaweed, they call for help.

Danielle Dixson and Mark Hay from the Georgia Institute of Technology have found that when Acropora corals detect the chemical signatures of seaweed, they release an odour that summons two gardeners – the broad-barred goby and redhead goby. These small fish save the corals by eating the toxic competitors. In return, one of them stores the seaweed poisons in its own flesh, becoming better defended against its own enemies.

Since as long as I can remember, nature documentaries and textbooks have said that flocking birds and shoaling fish gather in large coordinated groups to protect themselves from predators. That explanation makes complete sense. After all, many eyes can spot danger more easily, and many bodies can confuse the senses of hunters. But common sense often leads us astray in biology, and very few people have checked to see if collective motion does offer safety from predators.

Christos Ioannou is one of the first. By allowing a predatory fish to hunt virtual prey, whose movements he could precisely control, Ioannou showed that coordinated groups are, indeed, less likely to be attacked. So far, so obvious, but Ioannou’s results have a fascinating implication: predators can trigger the evolution of collective movements, even if their prey can’t see them. While we often think of flocking and shoaling as a responseto a hunter’s advances, that response doesn’t have to be a deliberate one. Threatened prey could evolve to move as one even without any knowledge of the threat they face.

I was at the Ecological Society of America’s Annual Meeting when I saw this tweet:

As you might imagine, I did check out that talk.

For those of you who are wondering how you weaponise shark teeth, which are already regenerating, serrated meat knives at the business end of a streamlined, electric-sensing torpedo, here’s how. You drill a tiny hole in them, and then bind them in long rows to a piece of wood to make a sword. Or a trident. Or a four-metre-long lance. And then, presumably, you hit people really hard with them.

That’s what the people of the Gilbert Islands have been doing for centuries. Sharks are an ingrained part of their culture and their teeth have been an ingrained part of their weapons. Tiger sharks feature heavily – they have thick, cleaver-like teeth that can slice through turtle shells so they make a good cutting edge. But the weapons also include the teeth from spottail, dusky and bignose sharks (you can identify species from their teeth), and none of these actually live around the Gilbert Islands today.

Anglers ensnare fish with bait, or with man-made lures that look like bait. Anglerfish do the same thing – they have worm-like growths on their heads that act as living fishing lures to entice their prey. The swordtail characin, a small fish from Trinidad and Venezuela, has a similar lure, and it uses it not to attract food, but sex.

The male characin has a small bean-shaped patch attached to his gill flaps by a thin thread. When he swims, he holds these ‘flags’ against his body. When he encounters a female, he flares one of them out in front of her. The female clearly thinks that the flag is food, for she bites at it vigorously. While she’s occupied, the male sidles across and impregnates her with his sperm.

Unlike many other fish, which shoot sperm and eggs into the water, characins fertilise each other internally, just like us. The male, however, doesn’t have any sort of penetrating organ, so he needs to bump into the female just so. And his bizarre ornament ensures that she’s in exactly the right place.

Niclas Kolm and Göran Arnqvist have been studying the characins for several years, and they’ve shown that characins from different Trinidadian streams have distinctly shaped flags. Now, they think they know why.

The characins feed on manna from heaven – insects that fall into the water from overhanging plants. On average, half of their diet consists of tree-dwelling ants, but that proportion can vary between 10 and 75 per cent. The ant portion of their menu is dictated by their environment: if they live in wider streams, they have more plants growing overhead, and more ants fall within their reach. Now, Kolm and Arnqvist have shown that the flags of male characins look more like ants in streams where the female eats more ants.

Together with Mirjam Amcoff and Richard Mann, they captured characins from 17 different streams around Trinidad. They measured the shape of the male’s flag and the contents of the female’s guts, and showed that these traits are related. In streams where females eat more ants, the males’ flags are more tapered and curved towards their far end. This more closely mimics the shape of an ant with its narrow waist connecting a thick torso and abdomen. It’s very different to the oval shape of a beetle – the characins’ second favourite food.

Are these ant-like flags more successful at attracting ant-eating females? Kolm and Arnqvist found out by using characins that had been raised in captivity, and had never seen an ant before. They fed these females with either ants or other insects, before presenting them with males from different streams. Sure enough, females that had gorged on ants were more likely to attack the ant-like flags of males from streams where females naturally eat lots of ants.

It’s a really elegant experiment – one that strongly supports the idea that the male flags have evolved to tap into the sensory biases of the females. As Kolm writes, “The shape of the male flag ornament…has evolved to track the search images that females employ in foraging.” It’s a lure that evolves according to the preferences of its target.

But the important thing here is that those preferences are originally driven by the environment. It’s the width of the streams that determines how many ants the females encounter, and thus what shapes the males’ lures take. This process, where animal signals evolve to account for the properties of their environment and stand out more strongly, is known as sensory drive. And in this case, it’s driving the divergence of different characin populations.

A migrating robin can keep a straight course even when it flies through a cloudy night sky, devoid of obvious landmarks. That’s because it can sense the Earth’s magnetic field. Something in its body acts as a living compass, giving it a sense of direction and position.

This ability – known as magnetoreception – isn’t unique to robins. It’s been found in many other birds, sharks and rays, salmon and trout, turtles, bats, ants and bees, and possibly cows, deer and foxes. But despite more than 50 years of research, the details of the magnetic sense are still elusive.

Unlike light, sounds or tastes, which come and go, the Earth’s magnetic field is always ‘on’. To study how animals sense it, scientists first have to cancel it out using magnetic coils, and set up their own artificial field. The field also pervades the entire body, so there’s no obvious opening, like an eye socket or ear canal, where a magnetic sensor would most likely lie.

In birds – the best-studied of the magnetic-sensing animals – scientists have narrowed down the location of a possible sensor to the eye (sometimes, just the right one), the beak, and possibly the inner ear. But it’s been far trickier to find the individual cells responsible for sensing magnetic fields. Now, Stephan Eder from the Ludwig-Maximilians-University in Munich has developed a way of doing that. It’s deceptively simple: look at cells under a microscope surrounded by a rotating magnetic field, and spot the ones that start to spin.

Some animals are poorly named. The flying lemur doesn’t fly and isn’t a lemur. The mantis shrimp isn’t a mantis or a shrimp. The killdeer couldn’t. But the giant bumphead parrotfish… it’s a giant fish with a beak like a parrot and a bump on its head. Nice one, biologists. You can have a point for that.

The giant bumphead parrotfish (Bolbometopon muricatum) is the biggest herbivorous fish in coral reefs. It can reach 1.5 metres in length and weigh over 75 kilograms, and it has a distinctively bulbous forehead. Why? There are rumours that it uses its head to ram corals, breaking them up into smaller and easier-to-eat chunks.

But Roldan Munoz from the National Oceanic and Atmospheric Administration has discovered one definite use for the bump: headbutting rivals. Check out the video below – it all kicks off at the ten-second mark, and I love the “Whoooah” that follows.

“Get off!” “No, YOU get off!” “Okay, we let go on three. One… tw…” “Wait, wait, is that one, two, three and then we let go, or one, two, let go?” “One, two, let go.” “Okay, okay. One… two…” And they both died.

Here is a fatal accident, etched in stone. This fossil comes from Solnhofen in Germany and dates back to the Jurassic period. On the left is Rhamphorhynchus. It’s a pterosaur – one of many flying reptiles that flapped through the skies while the dinosaurs ruled the land. Its arm bones, which supported its leathery wings, stretch out to the left of the image, while its long, stiff tail points downwards. On the right is Aspidorhynchus, a predatory fish with a long, pointed snout.

On first glance, you might think that the fish tried to eat the pterosaur. But Eberhard Frey and Helmut Tischlinger have been studying the fossil in detail, and they think otherwise.

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth. Sometimes, the slashes are powerful enough to cut a fish in half. Even less dramatic blows can knock a fish to the sea floor, and the sawfish pins it in place with its saw.

Albatrosses are superb long-distance fliers that can scour vast tracts of ocean in search of food. But sometimes, food comes to them. In July 2010, Tazuko Abe from Hokkaido University found albatrosses cleaning a school of ocean sunfish, basking at the surface of the western Pacific Ocean.

The ocean sunfish is a truly bizarre animal. It looks like someone cut the head off a much bigger fish and strapped fins to it. It’s the largest of the bony fish*. The biggest one ever found was 2.7 metres in length and weighed 2.3 tonnes. The youngsters, of course, are much smaller. The ones that Abe saw on his research cruise were just 40 centimetres long. There were at least 57 of them, each turned on its side so its broad flank faced the water surface.

The basking shoals were attending a sort of sunfish spa. The fish were infested with parasites. Pennella, a long scarlet relative of shrimp and crabs, was embedded headfirst in the flesh beneath their fins, busily sucking their blood. But not for long – black-footed and Laysan albatrosses were attracted to the shoal and picked the Pennella off their bodies. In some cases, the sunfish seems to be courting the birds, following them around and swimming sideways next to them.

Ocean sunfish live throughout the oceans but they often spend time at the surface before diving to the depths. Some scientists think that they’re absorbing heat from the sun, but it’s possible that they could also be looking for a spot of personal hygiene.

These fish can play host to at least 50 species of parasites, and they often have considerable numbers on their large bodies. Many ocean animals rely on cleaner fish or cleaner shrimp to rid them of parasites. It’s possible that albatrosses might fulfil the same role for ocean sunfish.

Of course, the association might have been a one-off. However, there are other reports of seabirds such as shearwaters and albatrosses flocking around schools of basking sunfish. This instance stands out only because Abe has photographic evidence that they were actually parasites. As he rightly points out, such events would be difficult to spot among the vastness of the open ocean.

* Fish have skeletons that are either made of cartilage, as in sharks and rays, or bone, as in all the others.